PGA TOUR LIVE: Thursday-Friday, 7:45 a.m. -6 p.m. ET (featured groups).
Johnson finished the day with one of the more colourful scorecards.
"Forty years old, they say things start breaking down a little bit, but hopefully this is pretty short-term and now I can go on for a few more years and enjoy some good golf", said Donald who topped the rankings for 56 weeks in 2011-12.
In 2012, when Donald was at his best, he won the Valspar in a four-way playoff. "Mentally previous year I was hoping I would win, wanting to win".
Dahmen already had a top finish this season when he tied for ninth place at the Farmers Insurance Open in January. Donald has fallen all the way to 919th in the world coming into this week, the time when he could say he was the best long gone. He has battled a back injury that forced him to miss most of last season and hasn't won anywhere since the Dunlop Phoenix Tournament in Japan in 2013. He's playing this year on a medical exemption, giving him 15 starts to earn enough FedEx Cup points to regain full-time status.
Bhatia bogeyed his last two holes for a 72 and finished at 4-over 146 to miss the cut.
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He spent the opening two rounds with Johnson, who had to maximize his birdies after his start.
"Of the guys on the leaderboard, he's the one I would want to be paired with, so it doesn't bother me", Casey said.
A number of golfers were close to not making the cut.
The World No. 1 sank a 33-foot birdie putt at the par-5 14th and curled in a tricky 10-footer for birdie at 18 to join Casey in the last pairing.
Casey carded an eagle, four birdies and a solitary bogey in a second round of 66 on the Copperhead Course at Innisbrook Resort to share the halfway with American Austin Cook on six under par.
The disappointing finish aside, Donald is taking nothing but positives from his play so far this week.
Indian golfer Anirban Lahiri opened with a creditable one-under 70 in the first round to sit in a tie for 28th place at the United States dollars 6.7 million Valspar Championship here. "I had a tough stretch in there, just kind of hung on, and then I hit some good iron shots on my back nine and, yeah, it was just a real solid out there today". He's had to endure so much and work so hard to climb back up, but now that he's on his way he's not ready to slide back down again just yet.